Three earthquakes that hit Dallas over the weekend were caused by past instances of hydraulic fracturing, according to Cliff Frohlich, associate director and senior research scientist at the University of Texas at Austin's Institute for Geophysics. (Photo: WikiCommons)

Three earthquakes that hit Dallas over the weekend were caused by past instances of hydraulic fracturing, according to Cliff Frohlich, associate director and senior research scientist at the University of Texas at Austin's Institute for Geophysics.

The first earthquake was a magnitude 3.4 on Saturday and was followed shortly by a magnitude 3.1. The area shook again on Sunday when it was hit by a magnitude 2.1.

Hydraulic fracturing, also known as fracking, is a method of extracting natural gas, the cleanest fossil fuel, from underground rocks. Proponents of natural gas say it could reduce the country's dependence on foreign oil since vast majority of it comes from within the country. Natural gas is also more plentiful and cheaper than oil.

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Workers use high-pressure fluids to create fractures in rocks such as sandstone and shale to allow natural gas to flow out. The injection of these high-pressure fluids, a combination of water sand and chemicals, are suspected of causing earthquakes when they rise back to the surface.

"That's dirty water you have to get rid of," Frohlich told LiveScience. "One way people do that is to pump it back into the ground."

Dallas had never recorded a magnitude 3 or stronger earthquake until 2008. But since then, the area has been hit by at least one every year except 2010. Frohlich said it's no coincidence that the earthquakes began a year after fracking began in the area.

"Faults are everywhere. A lot of them are stuck, but if you pump water in there, it reduces friction and the fault slips a little," Frohlich said, according to LiveScience. "I can't prove that that's what happened, but it's a plausible explanation."

The fracking well, which is just south of Dallas/Fort-Worth International Airport, has been out of use since September 2011, but Frohlich said that doesn't mean that lingering fluids cannot still contribute to seismic activity.

In a recent study by Frohlich, published in the Proceedings of the National Academy of Sciences, he analyzed 67 earthquakes recorded between November 2009 and September 2011 in northern Texas, and found that 24 of the earthquakes originated within two miles of one or more fracking sites.

Oliver Boyd, a USGS seismologist, told LiveScience that it's plausible and possible that the fracking is causing numerous earthquakes in the area.

"Most, if not all, geophysicists expect induced earthquakes to be more likely from wastewater injection rather than hydrofracking," Boyd said. "This is because the wastewater injection tends to occur at greater depth where earthquakes are more likely to nucleate. I also agree [with Frohlich] that induced earthquakes are likely to persist for some time (months to years) after wastewater injection has ceased."